BATS Global Markets, the exchange operator, said on Tuesday that its president, William O’Brien, had left the company.

The move, at a time when fast trading is under heightened scrutiny, comes less than six months after BATS completed its merger with the trading platform Direct Edge, where Mr. O’Brien was previously chief executive. The combined company became the second-largest exchange operator in the United States, passing the Nasdaq stock exchange.

Joseph P. Ratterman, BATS’s chief executive, has assumed the additional role of president, a position he previously held from June 2007 to January 2014, the company said in a news release. The company declined to comment on the reason for the management change.

Mr. O’Brien made waves in April shortly after the release of “Flash Boys,” the book by Michael Lewis about high-speed trading, when he sparred with Mr. Lewis and Brad Katsuyama, the book’s star, on CNBC.

“Michael and Brad, shame on both of you, for falsely accusing literally thousands of people and possibly scaring millions of investors in an effort to promote a business model,” he said at the beginning of the segment. The debate, which lasted about 20 minutes, quickly turned heated, riveting Wall Street.

“You had a 300-page commercial,” Mr. O’Brien said about Mr. Katsuyama’s new company, an upstart trading platform called IEX. Mr. O’Brien also questioned Mr. Lewis’s journalism and accusing him of dishonesty.

High-frequency trading has come under scrutiny by regulators, as electronic exchanges like BATS have increasingly taken over Wall Street. Of particular concern is that certain services allow fast traders to profit on information before other investors can see it.

Earlier this year, the New York attorney general, Eric T. Schneiderman, began a sweeping investigation into the stock market, focusing on fast trading. For their part, defenders of high-speed trading argue that it makes trading easier and less expensive for investors and increases liquidity.

A version of this article appears in print on 07/23/2014, on page B5 of the NewYork edition with the headline: Executive Departs.